Given the circumstances, the plan appears to be pretty solid to me. Given that there literally was no way for them to simply sneak past him, their plan to use the sheep hides as a way of tricking him into thinking that they are the sheep and will allow them out after he moves the stone, important since they cannot move the stone themselves.

Of course the additional part of the plot in making sure that they had referred to themselves as "nobody" became aparticularly vital once they had escaped fromo the cave and Polyphemus was calling for help.

The only part that was clearly stupid, wasn't necessarily part of the plan but when Odysseus decided he ought to taunt Polyphemus, he invited disaster.